I'm looking for an implementation of java.util.Map that has a method that will return all they keys mapped to a given value, that is, there are multiple keys map to the same value. I've looked at Google Collections and Apache Commons and didn't notice anything. Of course, I could iterate through the keyset and check each corresponding value or use two maps, but I was hoping there was something available already built.
+2
A:
I don't know if that solution is good for you, but you can implement easily that by using a standard map from keys to values and a MultiMap from values to key.
Of course you'll have to take care of the syncronization of the two structures, IE when you remove a key from the map, you have to remove the key itself from the set of keys mapped to the value in the multimap.
It doesn't seems difficult to implement, maybe a bit heavy from the memory overhead aspect.
akappa
2009-07-09 23:23:52
Thanks, yeah I thought about using two maps. It would be nice if there was an implementation in one map though so I wouldn't have to worry about keeping the two maps in sync.
Joel
2009-07-09 23:30:29
Looks like an implementation doesn't exist so this is the best way to go.
Joel
2009-07-10 02:17:09
+1
A:
What you're looking for here is a bidirectional map, for which there is an implementation in commons collections.
toluju
2009-07-09 23:30:32
Actually a bidi map won't work. I've edited the question to make it clearer, but there is no 1:1 mapping between keys and values.
Joel
2009-07-09 23:48:00
A:
Your value objects could have a property (of type ArrayList maybe) that holds all the keys. Then you extend HashMap (or whatever Map impl you use) and override put so that when you put and object for a key you also add the key to your object's list of keys.
nicktmro
2009-07-09 23:33:00
A:
I can't find a ready made class that supports values with multiple keys. However you could re-implement the Apache Commons DualHashBidiMap using a MultiHashMap in place of one of the HashMaps.
John
2009-07-09 23:36:20